Rocket Mass Heater for my home

[matt walker]

Lolly, the color is pigment for concrete work, I believe it's typically oxides of one sort or another. This one was yellow, but I added it to my brownish clay and got, uh, whatever it is. It's growing on me, but I'll do another layer at some point and try something else, maybe some red or something.

Guy, I think you have it right. The half barrels make a stove that will probably have less mass than a stove built around flue pipe, although as Graham noted it's all fairly adjustable depending on the depth at which they are buried and the size of barrel used. Typical install with the flue pipe is around 6" below the level of the top of the bench, so there's a lot of mass below them that doesn't get a lot of heating. I've helped a friend on a build in his small cabin using half 15 gallon barrels and there is quite a bit of mass over the top to reach finished height of around 18". So, yeah, it's part of the process of calculating how much thermal battery you want balanced with response time and floor load/space layout. If I had a small space I'd probably use flue pipe to maximize mass, but a half barrel system could be spread out over a very large area and give more radiant heat from the mass surface. Decisions, decisions.

[hpmer]

Matt,

Great stove! I'm really intrigued by your warming plate and have a few questions. I'm rebuilding a cooking stove I use for maple sugaring and usually have to use two stoves, one for preheating the sap and the second for the main boiling pot. Your hot plate would sure be convenient.

I seem to build a new stove every year improving on the prior year's design and this seems like a great feature to add.

Anyhow, assuming you're still happy with it, here are my questions:

1. What size pan did you use and did you have to cut off the handle?
2. Is the underside open to the fuel box or is it filled underneath with cob? If so, how much?
3. Any issues with different expansion rates between the cob and cast iron?
4. Anything you'd do different if doing it again?

I'm not sure what it is with these stoves but I find them a bit addictive...

[matt walker]

Welcome Hpmer, and thanks for the compliment. Yeah, I'm still happy with the hot plate, and I think it might work really well as a sap warmer. On this stove I used a standard 9" skillet with the handle still on. It's not open to the firebox, although that's an interesting idea. In this case it's just set on top of the core as it's being cobbed in, and I did pack the void of the skillet with cob as well. I think there's probably at least 4" of material, if not 6", between the skillet and the tunnel.

This stove has the warming plate out in front of the feed tube, so it's not right over the burn. I've built a batch box stove outdoors using a half barrel that had one right over the burn. It got real hot after a little while, such that I was able to hit a gentle boil on it fairly quickly. Not sure what temp you are going for but you might experiment a little with placement and thickness of perlite/clay underneath to achieve your desired temp.

For sure the different expansion rates are an issue. This example has actually stabilized, somehow, to where it is now encased firmly in the cob and hasn't developed any cracks for a long time now. When it was first built it was constantly developing new cracks around the skillet, which I would occasionally plaster over as I was maintaining my feed area, maybe once every two months. Like I said, it's stabilized now, but expect it to crack. It never created any major fault in the structure of the cob, just little expansion cracks around the perimeter of the skillet. I'd be inclined to not worry about it, especially for a sap burner, but I'm not sure what aesthetic you are going for. I did have a large slate stone that I would put over the whole are to protect the skillet from rain since it was outdoors.

Not much I'd do differently in regards to this idea, I have temporarily placed a cast iron griddle on top of the burn chamber of one of my half barrel batch burners, and that was really nice. It worked well for heating pots and pans, as well as cooking directly on it. I liked the griddle too much to sacrifice it for that use though. I'm not sure if they get wrecked, probably not, but they will develop surface rust where they contact the cob. I used CI I didn't really care about.

Yep, they are pretty dang addictive to me too. Let's see that sap boiler!

[hpmer]

Matt,

Not sure how to post pics or video, but I posted some pics over at Donkey's site a while back. I've got a video on my phone, but that's the only place it lives at the moment.

[hpmer]

Matt,

You've said that you exhaust your system directly into your home chimney. Did you simply stick the end of the exhaust pipe into it or did you run the pipe all the way up to the top of the chimney? I'm, concerned about the cooled gases not being able to sufficiently heat the mass of the masonry enough to form a draft to overcome the cold plug that might form.

My chimney serves a fireplace and sits on the end of the house outside and so is not a very well insulated entity. I'm wondering if I might even have to insulate the pipe all the way up the inside of the existing chimney. Do you have any idea what the temps of the gases entering and then exiting the chimney are in your system?

[pa_friendly_guy]

Hot air rises, as little as 10 degrees is enough to move air in your house from one floor to another. Before you go to the expense of adding more pipe why don't you try it in your existing chimney and see how it works. Just a thought, you can always add more pipe inside the flue later if you feel there is a problem.

[matt walker]

Hpmer, I don't know my temps very accurately. I would often use an IR gun on the flue pipe where it enters the wall, and it would typically read in the low 200°F range. I've read internal temps are roughly double outside with regards to flue pipe, but that seems high to me. I'm quite sure that internal temps are quite a bit more though, regardless. My flue pipe is just attached to the masonry chimney at the wall thimble, no liner.

The real question is how much do you intend to cool the gasses? In other words, how big a system, how much mass, what type of system, etc. I think it's most likely that you will build a system that reserves plenty of heat for the chimney, but it's possible to take out too much heat for a chimney like you describe, for sure. I personally would do as Guy suggests, but on the other hand, if installing an insulated liner in the chimney is an option, it's always a good idea.

[hpmer]

It won't be a very big system at all. I was just reading a masonry heater book that strongly cautioned that the chimney should match the system otherwise there would be all matter of problems such as smoke back, creosote, etc.

I'm thinking of building a system using the footprint of the fireplace plus a little more, so basically the burn unit plus a SMALL bench since there is very little available real estate to use.

Probably does make sense to try it and see. I'm just trying to get a good handle on the issues first.

[Oddmar]

Here's a design i've come up with for a friend's house...we'll be building this for his primary (only) heat source this winter. The concrete columns are made from concrete foundation block with liquid concrete and gravel filling the holes in the block. You could use red brick mortared together i guess. I'd put steel plates over the top as cleanout access. Both concrete "bells" have ash pans in the bottom, even though the primary bell (on the right) is the only one that should need cleaned out.

It is built to look like a regular fireplace, the back of the 'firebox' is just a tin sheet and rests against the drum. I'll build a small fan into the exhaust pipe to prime the system for easier lighting, but in case of a power outage the Tee is hidden behind a thin 'rock' in the rock facade. I will probably build in a cold-air intake, and maybe the usual glass doors you'd find on a normal fireplace. I was thinking of thermo-ceram glass built into the front of the feed chamber so the fire would be visible.

[matt walker]

Very cool design Oddmar, can't wait to see it come to life.